ABSTRACT. As recognized in the NIH INCLUDE (Investigation of Co-occurring conditions across the Lifespan to Understand Down SyndromE) Project, it is well-established that trisomy 21 (T21) predisposes people with Down syndrome (DS) to develop diverse leukemias, while also protecting them from most solid malignancies. However, the mechanisms by which T21 achieves these effects are poorly defined. Recently, members of the University of Colorado Cancer Center (UCCC) discovered that T21 consistently activates the interferon (IFN) response in multiple cell types, concurrent with proteomic and metabolomic changes consistent with IFN hyperactivity in people with DS, which could be explained by the fact that four of the six IFN receptors (IFNRs) are encoded on chromosome 21. Importantly, IFN signaling is a potent regulator of hematopoiesis, immune activation, and tumor suppression. Therefore, the goal of this supplement to the UCCC Cancer Center Support Grant (CCSG) is to test the paradigm-shifting hypothesis that hyperactive IFN signaling is a main driver of the differential malignancy spectrum observed in people with DS. More specifically, we hypothesize that hyperactive IFN signaling disrupts hematopoiesis, predisposing individuals toward a leukemogenic state by promoting clonal expansion of cells bearing specific mutations, while also suppressing the development of malignancies through a combination of tumor-intrinsic and -extrinsic mechanisms, such as immune activation. Furthermore, we posit that increased IFN signaling augments the toxic side effects of chemotherapy, while also impacting on the function of chimeric antigen-receptor T cells (CARTs). Funded by this supplement, the UCCC will assemble a multidisciplinary and synergistic team of UCCC members who will test these hypotheses through complementary studies in humans and mice. Enabled by key CCSG- supported Shared Resources, this team will use human samples obtained from an ongoing pan-omics cohort study of people with DS and novel mouse models of DS carrying varying copy numbers of the IFNR gene cluster to define the role of IFNR triplication and hyperactive IFN signaling on: Clonal expansion of leukemogenic mutations in the hematopoietic compartment Increased toxicity during chemotherapy for pediatric leukemias Protection from carcinogen- and oncogene-driven solid malignancies Tumor surveillance by Natural Killer and T cells These investigations clearly address Component 1 of the INCLUDE project (high risk-high reward basic science), while also leveraging available resources related to Component 2 (pan-omics cohort study of DS). Furthermore, the research proposed falls well within the scope of the parent award in particular, and NCI more broadly, by investigating mechanisms that could lead to novel strategies for cancer prevention, diagnosis, and treatment.